Storage of urine at −20° was found to inactivate γ-glutamyl transpeptidase in many samples. The inactivation was more pronounced in concentrated samples than in diluted ones, and was probably related to the amount of electrolytes present. Dialysis or the addition of dimethylsulfoxide or glycerol before the freezing protected the enzyme. An addition of 5% (v/v) of dimethylsulfoxide was found to be satisfactory to completely counteract the inactivation. In contrast to what has been reported earlier, the γ-glutamyl transpeptidase activity of fresh urine samples did not show any increase after dialysis.